Blind Hole Depth

[[Ma...]]

This will show you how to get a blind hole depth. This is an excerpt from the Calypso Handbook

https://cmm-quarterly.squarespace.com/a ... hole-depth

[[Th...]]

I used this to measure a blind drilled hole once and got in some trouble for it because the angle of the drill point changed halfway along its surface. Ended up incorrectly calculating the depth as nonconforming. This was only a ~2mm bore though.

[[Mi...]]

This is the same method Zeiss recommends, it's difficult to explain and justify without giving a math lesson, especially if you don't have the two common angles.

For anybody who has CAD software available it's much easier to do this. In CAD, create a simple calculator, here is a 3mm ruby being used to calculate the depth of a blind hole with a 133.xxx degree cone angle (In this case the depth was really important so I measured the angle on a couple parts with a tracer, then CMM'd the rest of the parts I had, normally I would assume the perfect angle like the Zeiss method does)



In Calypso, measure the distance from your self centered point set to Midpoint evaluation, and then add 0.726627 to the result via a result element.

This method goes over much better in meetings. Both options assume the angle, and they correlate really well, this is just way easier to explain to managers.

[[Ri...]]

Thanks, Michael.

Pretty dangerous to assume an angle, and it's much safer to verify the angle (if you can) and then do the trig. Also, a lot of drawings don't even callout the nominal angle for the drill point, and most likely don't care if the machinist uses a 118 or a 135, so I wouldn't just base it on the model nominal.

Tool makers themselves have a tolerance on their tools. The first Google search for tolerances showed a ±5° allowance.

In Michaels example, that angle allowance equates to approximately ±0.073mm worth of height variance. (This changes depending on the size of the drill hole, so it can possibly be significantly higher than this)

Use at your own risk.

[[DW...]]

Interesting. I don't recall running across any blind hole depths that were that serious.

Most hole depths I have seen are min depth call outs, no max (usually a note not to go through). Because great accuracy is usually not required, I use calipers or a height gage with a cylindrical probe to catch the shoulder.

[[Ri...]]

Please sign in to view this quote.

Matters the most when they make it on the limit. 🙂

[[Mi...]]

Please sign in to view this quote.

100% agree. Anywhere the hole is big enough and drill point well formed enough I would rather measure the cone and intersect it with the wall. Or lines and again intersect with the wall.

The example I used was a stupid set of prototype parts for an EV that never happened with a 0.025mm tolerance that some engineer agreed to. They also required CMM results as they didn't like any manual method we offered other than tracing, and it wasn't practical to trace that many parts with this hole pattern. I traced a few to get the angle and was promised it wouldn't change throughout the batch of drills we had(ha).

That 133* drill point angle was supposed to be 130* which definitely agrees with your statement of its dangerous to assume the angle. Both of these methods should be used with caution. That one was just too small diameter and too deep to measure more correctly.

All that said, a drill point is usually a pretty crappy surface so the results with other methods tend to be less stable than you'd like, even on larger holes. Sometimes the stability of a self centered point is better than being completely correct in your method. It may not be totally right, but at least you'll keep getting the same answer.

[[Ma...]]

The intent is to show the capability and the formula to calculate the result. I do agree that any results should be verified while targeting the hole and the machining process. That just makes good sense, but this method will work if you refer to the bore and stylus table.